For die bonding during the assembly of electronic packages, a robust die attach process is critical to achieve low electrical resistance, low thermal resistance and good mechanical and electrical integrity. The existing die bonding processes for High Brightness LED (HBLED) devices typically involve silver (Ag) epoxy bonding, gold-tin (AuSn) eutectic bonding, thermosonic (TS) bonding with gold (Au) studs or solder reflow with solder bumps for flip chip bonding. Although silver epoxy bonding is a simple and mature process, its low thermal conductivity has limited its application in high power applications. The flip chip structure has the drawback of having a relatively small contact area for effective heat dissipation. Among these die bonding methods, gold-tin eutectic bonding offers the advantages of low thermal resistance and a relatively large contact area which are especially beneficial for power device applications. During chip fabrication, an 80% gold/20% tin eutectic metal layer is deposited on the bottom of the chip. The melting temperature of this metal layer is typically about 280° C.
For gold-tin eutectic die attachment, there are currently two possible approaches, namely flux eutectic die attach and direct eutectic die attach. During flux eutectic die attachment, a small volume of flux is placed on the package substrate, and the LED is placed onto the flux. After that, the substrate with multiple LEDs mounted on it will be put into the reflow oven to complete the bonding. There is no external force applied throughout the process. The advantage of this method is that no squeeze-out of the die attach metal occurs. However, there are some problems which retard the effectiveness of the process. Of most concern is flux residue, which might lead to package reliability issues due to moisture corrosion. Furthermore, chip movement including die tilting and die rotation cannot be avoided due to the difficulty of controlling the flux dispensing volume and uniformity with precision. Insufficient flux induces non-wetting of the gold-tin material, but using too much flux causes poor wetting of the gold-tin material and affects placement accuracy.
Direct eutectic die attach involves preheating the substrate to 300-320° C. in an ambient chamber with shielding gas, then an LED is picked by a bond head collet and placed onto the heated substrate with a compression force. After a certain time (about 100 to 200 ms), the contact force is released. Initially in this process, the gold-tin eutectic layer will be in a molten state on the substrate. After the substrate bond pad materials (gold, silver, palladium, etc.) dissolve into the molten gold-tin layer, and reach the saturation limit at this temperature, solidification will occur due to the high melting point of the off-eutectic composition. As a result, the LED is bonded on the substrate by the gold-tin eutectic material. Since an external force is utilized during the said LED die attachment, and no flux is needed during this process, the bonding performance has been found to be more encouraging as compared to flux eutectic die attachment.
However, two issues have emerged recently for direct die attachment during the manufacture of HBLED devices. Firstly, due to the slow solidification rate of the thick gold-tin layer, the squeezed out solder tends to flow back after the bond head is removed from the chip surface, and voids may appear at the interface between LED and the substrate after bonding. The voids are unacceptable, since the reliability of packaged electronics strongly depends on the die attach quality, and any void or a small delamination may cause instant temperature increase in the die, leading sooner or later to failure in the package. More importantly, with the increase of substrate size, the number of the units to be bonded onto each substrate reaches several hundreds or even over one thousand units. It will take a significant amount of time to finish bonding LEDs onto the whole substrate, and the LEDs already bonded will endure a longer annealing time on the heated substrate, which will degrade the LED performance.
It would be desirable to adequately accelerate the solder solidification speed to control the appearance of voids in the interface. It would also be desirable to avoid the annealing of LEDs which have already been bonded onto a heated substrate, leading to the degradation thereof.